The British government may have been complicit in ill-treatment of Michael Adebolajo, one of Lee Rigby’s killers, during his detention in Kenya, the intelligence and security committee has said.

The official inquiry into the circumstances leading up to Rigby’s murder also criticised evidence from the former MI6 chief Sir John Sawers, saying it was incorrect and that a document showing this was initially not given to the committee by the intelligence service.

Adebolajo alleged that he was beaten and on more than one occasion threatened with electrocution and rape during his detention in Kenya in 2010. The ISC said he had gone there to join extremists in neighbouring Somalia.

Britain has a close relationship with Kenyan anti-terrorism forces and has paid for some of their facilities. Amnesty International described as “worrying” claims of possible British complicity in torture.

The ISC said Adebolajo was interviewed first by Kenyan police and then by a counter-terrorism unit, the ATPU, whose name the committee redacted, using the term Arctic instead. “This unit has a close working relationship with HM government (HMG),” the report says.

It says MI6, also known as the Secret Intelligence Service, did not try to find out which Kenyan unit he claimed had tortured him. “This is surprising: if Adebolajo’s allegations of mistreatment did refer to his interview by Arctic then HMG could be said to have had some involvement – whether or not UK personnel were present in the room,” it says.

The ISC added in a paragraph partially redacted for national security reasons: “Given that *** has a close relationship with Arctic, ***, this certainly could be enough to raise questions of complicity.”

The report concludes: “Where HM government has a close working relationship with counter-terrorist units, they will share responsibility for those units’ actions. HMG must therefore seek to ensure that the same legal and moral obligations to which HMG adheres, and guidance which they follow, also apply to such units.

“Where there is a possibility that an allegation of mistreatment might refer to a unit where HMG has such responsibility, then HMG must investigate as a matter of priority to establish whether the unit is involved.”

The report says Sawers told the committee that MI6 had no responsibility for enquiring into Adebolajo’s claims. It quotes Sawers as saying: “[The committee is] suggesting that somehow we should have treated this as an SIS responsibility, when it is simply not the case. It is not an SIS responsibility.”

But an email from an MI6 officer contradicted this, saying: “We obviously need to investigate these allegations, which underline the need for continuing assurances from Kenyans on the issue of detainee treatment. We would be grateful if you could provide a summary of [HMG] and [Arctic] involvement in the investigation into Adebolajo …”

The ISC said this undermined the evidence of Sawers, who announced his departure from MI6 in October by which time copies of the committee’s findings were circulating in security and government circles.

“This email clearly indicates that SIS officers believed that they had a responsibility to investigate the allegations made by Adebolajo, particularly in light of ***. We support this view. However, SIS has no record of any response to this email and it is not consistent with the evidence provided to the committee by the chief of SIS …” the report says.

Furthermore, the committee said, MI6 did not initially hand over the email. “We note our concern that this email was not provided as part of the primary material initially offered in support of this Inquiry as it should have been. It was clearly relevant to the issues under consideration,” it said.

The report says MI6 dismissed the Adebolajo’s claims as “fabricated” without asking the Kenyans about them. “The committee has been concerned about the way SIS dealt with Adebolajo’s allegations of mistreatment. SIS does not seem to have taken them seriously, even in the wake of previous allegations.”

The ISC said it would not confirm or deny claims that MI5 had been trying to turn Adebolajo into an informant.

The committee did not take evidence from those to whom Adebolajo complained about his alleged ill-treatment in Kenya.

Amnesty’s Tom Davies said: “It’s worrying that the ISC raise the question of whether the UK’s security services may have been complicit in the torture of Michael Adebelajo in Kenya. The murder of Fusilier Lee Rigby was an utterly atrocious act, but that doesn’t in any way excuse the UK’s security services if they were involved in the torture of a British national overseas.

“The UK’s obligation to properly investigate allegations of state involvement in torture applies to everyone, no matter how abhorrent their actions. Turning a blind eye to allegations of torture is never justifiable.”

A government spokesman said: “We are confident that we have strong systems in place to prevent any complicity in mistreatment. The UK stands firmly against torture and cruel, inhuman and degrading treatment or punishment of individuals, whether held in the UK or overseas.

“SIS investigated the allegation when they became aware of it, but they do accept that they should have recorded the outcome of that investigation. All relevant SIS staff are trained on handling these issues and understand their responsibilities.”